Revanchism (from French: revanche, "revenge") is a term used since the 1870s to describe a political manifestation of the will to reverse territorial losses incurred by a country, often following a war or social movement. As a term, revanchism originated in France in the aftermath of the Franco-Prussian War among nationalists who wanted to avenge the French defeat and reclaim the lost territories of Alsace-Lorraine.

Revanchism draws its strength from patriotic and retributionist thought and is often motivated by economic or geo-political factors. Extreme revanchist ideologues often represent a hawkish stance, suggesting that desired objectives can be achieved through the positive outcome of another war. It is linked with irredentism, the conception that a part of the cultural and ethnic nation remains "unredeemed" outside the borders of its appropriate nation-state. Revanchist politics often rely on the identification of a nation with a nation-state, often mobilizing deep-rooted sentiments of ethnic nationalism, claiming territories outside of the state where members of the ethnic group live, while using heavy-handed nationalism to mobilize support for these aims. Revanchist justifications are often presented as based on ancient or even autochthonous occupation of a territory since "time immemorial", an assertion that is usually inextricably involved in revanchism and irredentism, justifying them in the eyes of their proponents.

French revanchism influenced the Treaty of Versailles following the end of World War I, which regained Alsace-Lorraine for France and extracted reparations from the defeated Germany. The conference was not only opened on the anniversary of the proclamation of the Second Reich, the treaty also had to be signed by the new German government in the same room, the Hall of Mirrors.

Ethnographic Lithuania was an early 20th-century concept that defined Lithuanian territories as significant part of the territories that belonged to Grand Duchy of Lithuania and Lithuanians as all people living on them, regardless of whether those people spoke Lithuanian language and considered themselves Lithuanian. The concept was in contrast to those of "historic Lithuania" - the territories of the Duchy - and the "linguistic Lithuania", the area where Lithuanian language was overwhelmingly spoken.

The concept of ethnographic Lithuania clashed with the right for self-determination of people living in that large territory, particularly Poles and Belarusians, who according to the supporters of the ethnographic Lithuania, were "slavicized Lithuanians" who needed to be re-Lithuanized. They argued that an individual cannot decide on his ethnicity and nationality, that it is not related to the language but to their ancestry.

In the 1920s and '30s, Poland was trying to reclaim ethnic Polish lands that had been occupied by German, Russian and Austro-Hungarian empires. According to Richard Overy in The Road to War: The Origins of World War II:

"Poland counted herself among the revisionist powers, with dreams of a southward advance, even a Polish presence on the Black Sea. The victim of the revisionist claims of others, she did not see the Versailles frontiers as fixed either. In 1938 when the Czech state was dismembered at the Munich conference, Poland issued an ultimatum of her own to Prague, demanding the cession of the Teschen region; the Czech government was powerless to resist." p. 28

The annexation of the Crimean peninsula by the Russian Federation in April 2014, together with accusations by Western and Ukrainian leaders that Russia is supporting separatist actions by ethnic Russians in Ukraine's Donbas region, has been cited by a number of prominent media outlets in the West as evidence of a revanchist policy on the part of the Kremlin and Russian President Vladimir Putin.[3][4]